Thanks David. It's fixed now. What's impressive is the care that goes into these old instruments even though they are still manufactured. Follow the link in the first paragraph and see the rest of the story.
Hi Martin...I can understand tubes in a megohmmeter, for the high impedance, but you'd think they'd be able to transitorise a Tach. Then again, if it works (and you can still get the tubes) don't fix it....
That's the thing about test equipment. Not only does "If it works, don't fix it" apply, but most people buying these already have them. They know what to expect from the equipment. Even if you can make it better, you don't. Well, not always. Why? Because having something better might reveal faults that would otherwise go undetected. Igorance is bliss.
There's a few Russian companies that manufacture vacuum tubes, some even make them to order. A friend of mine wanted to make a valve amplifier that actually produced high quality audio and designed a valve that met hit criteria. He's using one of these companies to make it :-)
One thing surprises me is that how they are able to maintain their staff or train their new staff. As the fresh grads coming out of colleges wouldn't 'have seen something called Vacuum tube, they need to have some retrograde training on basics of electronics and components of the older generation.
On the top of my cube wall is perched a 6201 / 12AT7 dual triode for all the younger staff to see. They may not know how it works, but they can now say they have actually seen a real vacuum tube (valve).
The old test equipment just keeps on going - and going - and going. No flash memory to degrade and guarantee failure in just a few years.
@prabhakar_deosthali, that may be the one thing that puts a company like IET out of business. Everyione who works there is an expert at his or her job and has been doing it for many years. When they reture, who will replace them?
There's another issue and that is because these people have a unique skill that's not necessarily transferable to other jobs. Thus, they have a job for life as long as the company stays in business, but there's little room for advancement or for changing jobs.
When I was a TA in charge of the physics lab, we had a closet with old but working equipment, including some voltmeters that were made in the thirties. What really impressed me though is the note inside stating that it was made to a design by Lord Kelvin from 1870s. That's longevity.
I worked on a lot of old equipment, and it all had the same problem, it was unreliable, drifted like crazy and NEDED calibration to attain what are now considered mediocre specs. These days any voltmeter from a company like Fluke etc. that is ~1% accurate usually doesn't need calibration, we just do it because the standards say so. Get to 0.1% and you occassionally need calibration. Things sure have improved
The caption says there's a CRT in the upper right corner of the capacitance bridge panel in image 13, but is it really? I have a hunch it's just a little screen over the top of a 6E5 "magic eye" tube but I could be wrong, those were quite common on test equipment of that vintage and function. Basically it's a replacement for a meter when all you needed was an indication of an analog null condition, like showing that the bridge was properly in balance.